The present invention relates to a method for reading data recorded in a high-density optical memory including several levels of data. One of the objects of the present invention is to increase the signal-to-noise ratio in order to reduce the read error rate.
A system which allows a very high storage capacity in a digital optical disk is disclosed in the French patent application No. 90 07284 filed on Jun. 12, 1990 by the present applicant and entitled "Stockage optique d'informations en strates superposees" (Optical storage of data in superposed layers). In this system, in order to increase the number of recorded data with respect to conventional optical memories, the data are coded as bits, in the form of a local change in the index of refraction or a local change of the absorption rate--which in itself is known--but on several levels in the volume of the optical disk made of a transparent material, and no longer only on the surface as this is known. This is consequently a three-dimensional recording system made up of several planes within the thickness of the disk.
Reading the data stored on several levels is performed by optical means, by diffraction or absorption of a light beam, and the requirement for a short depth of field to read a single level at a time, implies that the aperture of the light beam be wide and, as a result, the light beam also scans data contained in planes higher and lower than that being read. The parasitic bits, even though they have a low diffraction, cause a significant background noise and a poor signal-to-noise ratio.
This is the disadvantage the present invention is intended to remedy. The proposed solution consists in modulating the position of the focusing point of the read beam and in performing a detection at the modulation frequency. The position modulation can be parallel to the read plane, or perpendicular: in both instances, only the bit adjacent to the focusing point of the read beam--thus the useful bit for the read operation--has its diffracted power changed in a significant manner, while parasitic bits produce the same overall background power. Scanning the position of the useful bit results in the recovered signal to be a periodic signal.